Demand for lithium secondary batteries as energy sources is rapidly increasing as mobile device technology continues to develop and demand therefor continues to increase. Recently, use of lithium secondary batteries as a power source of electric vehicles (EVs) and hybrid electric vehicles (HEVs) has been realized. Accordingly, research into secondary batteries, which may meet a variety of requirements, is being actively performed. In particular, there is high demand for lithium secondary batteries having high energy density, high discharge voltage, and output stability.
In particular, lithium secondary batteries used in hybrid electric vehicles must exhibit great output in short time and be used for 10 years or more under harsh conditions of repeated charge and discharge on a daily basis. Therefore, there are inevitable requirements for a lithium secondary battery exhibiting superior stability and output characteristics to existing small-sized lithium secondary batteries.
In connection with this, existing lithium secondary batteries generally use a lithium cobalt composite oxide having a layered structure, as a cathode and a graphite-based material as an anode. However, LiCoO2 has advantages such as superior energy density and high-temperature characteristics while having disadvantages such as poor output characteristics. Due to such disadvantages, high output temporarily required at abrupt driving and rapid accelerating is provided from a battery and thus LiCoO2 is not suitable for use in hybrid electric vehicles (HEVs) which require high output. In addition, due to characteristics of a method of preparing LiNiO2, it is difficult to apply LiNiO2 to actual production processes at reasonable cost. Furthermore, lithium manganese oxides such as LiMnO2, LiMn2O4, and the like exhibit drawbacks such as poor cycle characteristics and the like.
Accordingly, a method of using a lithium transition metal phosphate as a cathode active material is under study. The lithium transition metal phosphate is broadly classified into LixM2(PO4)3 having a NaSICON structure and LiMPO4 having an olivine structure, and considered as a material having superior stability, when compared with existing LiCoO2. At present, Li3V2(PO4)3 having a NaSICON structure is known and as compounds having an olivine structure, LiFePO4 and Li(Mn, Fe)PO4 are the most broadly researched. However, due to low electron conductivity of LiFePO4, internal resistance of a battery increases when LiFePO4 is used as a cathode active material and thus polarized potential increases when battery circuits are closed, thereby resulting in reduction of battery capacity.
Meanwhile, a carbon-based active material is mainly used as an anode active material. The carbon-based active material has a very low discharge potential of approximately −3 V, and exhibits extremely reversible charge/discharge behavior due to uniaxial orientation of a graphene layer, thereby exhibiting superior electrode cycle life. An electrode potential of the carbon-based active material is 0 V (Li/Li+) during charging of Li ions and thus may exhibit a potential similar to pure lithium metal. Accordingly, greater energy may be obtained when a cathode and a battery including a lithium transition metal oxide are formed.
Examples of the carbon-based active material include crystalline graphite such as natural graphite, synthetic graphite and the like, and amorphous carbon such as soft carbon, hard carbon and the like. The crystalline graphite has high energy density but relatively poor output characteristics, thereby being unsuitable for energy sources for hybrid electric vehicles (HEVs) requiring high output. In addition, when an ester based material is used as an electrolyte, there is a problem such as decomposition of an electrolyte.
Therefore, a lithium secondary battery meeting all of the characteristics such as high output, long cycle life and conservation lifespan, high stability, and the like is preferred as a second battery for hybrid electric vehicles (HEVs). However, such a lithium secondary battery is still under development.